A number of projects are suggested to gain a clear understanding of Toxoplasma gondii and toxoplasmosis. Certain of these proposals are designed to study obligate intracellular parasitism by exploring the molecular biology of Toxoplasma in its intracellular and extracellular form. Studies of the immunology of the parasite will be carried out to define its antigenic nature as well as the immune response of the infected animal and human host. The mechanism of transmission of Toxoplasma in the acquired and congenital forms of the infection will also be studied. In addition, an attempt will be made to determine the relationship of this parasite to a variety of clinical disease states, including human abortion and perinatal fetal mortality. The very important area of infection in the compromised host (organ trnsplantation, cancer, immunosuppressive therapy, etc.) will be studied using Toxoplasma as a tool for the study of resistance to intracellular infection so common to these patients.